Windows: Fix declaration for C99 and re-enable Gnu.C test.

[marsupial]

The test was failing for a reason that likely shouldn't be ignored.

This reverts commit 5947e13.

[bellenot]

And it's still failing...

[marsupial]

Hopefully no longer.

[bellenot]

Remove C++11 test which Visual Studio cannot handle.

And what about the other platforms?

[marsupial]

The test is for GNU keywords, having -std=gnu++14 -std=gnu++1z is enough as they test the same mechanism.
-std=gnu++11 really adds nothing but failure for Windows, and fixing for that would mean you're just testing that C++11 cannot run on Windows

If someone feels it is really necessary, then it would probably be best to have a directory devoted to C++11 tests that are not activated on Windows rather than allowing failure all over.

[bellenot]

OK, thanks for the explanation @marsupial. If @vgvassilev confirm (and agrees), and if it fixes the test, then it is would be fine with me

[marsupial]

Updated again to work-around the Windows issue without dropping it entirely.

[vgvassilev]

Looks good to me.

[pcanal]

@phsft-bot build

[ferdymercury]

Moved to root-project/root#16694